1. Field of the Invention
The invention relates to circuits which permit the connection of a telephone designed for non-party line use to a party line of the type which selectively rings subscribers by the generation of any one of a plurality of specified ringing frequencies.
2. Description of the Prior Art
Telephone companies have developed party line service which selectively rings different subscribers assigned to a single line by the generation of a single ringing frequency assigned to each telephone subscriber. Each subscriber's telephone is provided with a ringing circuit which is tuned to resonate at the particular ringing frequency which had been assigned to that customer. The tuning of the subscriber's ringing circuit to resonate at a particular ringing frequency automatically rejects other ringing frequencies. With the party line service of the frequency selective type as set forth above, it is necessary for the telephone company to provide each subscriber with a telephone with a properly tuned ringing circuit to respond to the party line ringing frequency assigned to the subscriber by the telephone company central office. Party line telephones of the above-described type were provided for each customer as a part of the monthly rental fee for party line service from the telephone company.
Three combinations of party line ringing frequencies, known as harmonic, synchromonic and decimonic, are conventionally used by different telephone company central offices. The distinct ringing frequencies in the harmonic system are 16.66 Hz, 25 Hz, 33.33 Hz, 50 Hz and 66.66 Hz. The distinct ringing frequencies in the syncromonic system are 16 Hz, 30 Hz, 42 Hz, 54 Hz and 66 Hz. The distinct ringing frequencies in the decimonic system are 20 Hz, 30 Hz, 40 Hz, 50 Hz, and 60 Hz.
Party line ringing systems which use tuned ringing circuits are described in various United States Patents. U.S. Pat. No. 3,005,053 discloses a system for ringing substations by the transmission of particular tones applied in common to each of the substations. Each substation is selectively ringable by having a tone ringer which is tuned to a different resonant frequency. U.S. Pat. No. 3,026,377 discloses a party line ringing system which utilizes tuned relay circuits which resonate at a distinct resonant frequency at each subscriber station to permit the selective ringing of a subscriber station by a particular party line ringing frequency at the resonant frequency of that subscriber station.